This invention relates to photovoltaic generators for producing a control signal for the control of a semiconductor switching device and, more specifically, relates to novel photovoltaic generator structures and methods for making such structures in which an array of planar cells formed in a silicon substrate are dielectrically isolated from one another by trench isolation.
Photovoltaic generators (PVG) are well known and are commonly used for producing a control signal for a solid state relay. Such devices employ an LED which is energized by input terminals to irradiate the photosensitive surface of a spaced and insulated photovoltaic device. The output of the photovoltaic device may serve as the input to a switching device, such as a MOS-gated device, typically a power MOSFET, which has load terminals which are switched "on" in response to the energization of the LED. The input and output terminals of the relay are isolated by the gap between the LED and the photovoltaic device. Commonly, the photovoltaic device consists of a large number of series-connected photovoltaic cells in order to produce a voltage sufficiently high to turn on the power switching device. Such devices are well known and are sold under the name "PVI" (photovoltaic isolator) by the International Rectifier Corporation of El Segundo, Calif., the assignee of the present invention.
Plural cell photovoltaic generators can be made in many different ways. U.S. Pat. No. 5,549,762, issued to William F. Cantarini, one of the present inventors, on Aug. 27, 1996, the disclosure of which is incorporated herein by reference, describes a PVG device and method for making the same in which an array of planar cells 2 formed in a silicon substrate 4 are dielectrically isolated from one another by trench isolation. FIG. 1 shows a top view of such a trench isolated PVG device including contacts 6 to connect the cells 2 in series. FIG. 2 shows a cross-sectional side view of the device of FIG. 1 which includes polysilicon-filled trenches 8 which isolate the cells of the device.
To provide enough output current from a photovoltaic generator (PVG), sufficient surface area and depth of silicon need to be exposed to radiation. Experimentation and calculations have shown that greater than 80% of the incident light (typically 880 nm) can be absorbed in 20 nm of silicon. Reduction of this depth dramatically affects the absorption and hence output current. To maximize output current, it would be desirable to form a trench-isolated PVG device with increased absorption of incident light.